1. Field of the Invention
This invention relates to the making of forming tools, in particular injection molding tools such as are used to define one or more surfaces of a cavity for molding a part made of plastics material, e.g. a trim or body part to be used in a motor vehicle.
The term "forming tools" is intended to include without limitation, injection molding, blow molding, press, hot press, and die casting tools.
In this specification the following abbreviations are used:
CAD: computer aided design; PA1 NC: numerical control or numerically controlled, according to context; PA1 EDM: electrical discharge machining.
2. Description of the Prior Art
One known method of making an injection molding tool is by NC machining from a rectangular block of material. For example, to produce plastic parts, such as a bumper of an automobile, this may be carried out according to the following process. A positive master model is made from wood and resin materials which conforms to the shape of the article to be manufactured by the tool. The master model is copied into a metal tool blank using pantographic machining techniques to provide one part of the tool. The positive master model is then provided with a cover layer of a suitable material, typically glass reinforced plastic, which when removed from the master model, provides a "negative" model of the article for manufacture. The negative model will therefore have a surface which has a shape corresponding to that of the article for manufacture. The shape of this surface is then copied into a second metal tool blank also using pantographic machining techniques to provide the cooperating part of the injection molding the tool. The tool blanks are usually of aluminum alloy or steel depending upon the number of articles to be produced by the injection molding tool. As will be appreciated from the above process, as co-operating parts of the tool are machined from the tool blanks by NC (numerically controlled) machining, with possibly some finish machining using electrical discharge machining (EDM) techniques, the machining time for large forming tools, such as a tool for injection molding an automotive vehicle impact bumper, can be in the region of two or three months. To this must be added the time taken to produce the wood and resin master model which, as the materials are difficult to work, can take, typically, six to twelve weeks to manufacture. The manufacturing cost of forming tools for relatively large and complex shapes (such as a vehicle impact bumper) is therefore, extremely high and the forming tool can take, in total, nine months to produce if this conventional fabrication process is adopted.
Another known method is to machine the tool from a rectangular block of material, using conventional EDM techniques, which is an extremely time consuming and expensive procedure, often requiring several weeks of continuous EDM machining to achieve the desired shape with the attendant high production costs.
In order to avoid these complicated and time consuming processes, it has, therefore, been considered by the applicant to fabricate forming tools using a casing process in which a tool blank is cast in a predetermined shape which approximates to the final tool shape and an EDM process in which the tool blank is subjected to certain EDM operations.
Casting molds for use in fabricating cast metal shapes are usually produced by forming a slurry of sand or a sand/resin mixture around a casting model. The casting model is placed within a frame and the mixture is compacted into the space between the frame and the model and allowed to set. For large forming tools the sand mixture casting mold must have sufficient inherent strength to support the weight of the cast material, which can be in excess of 10 t. The mixture is therefore compacted around the casting model using high density ramming techniques. In view of the compaction forces required, the casting model must have a sufficient rigidity so as to maintain the desired shape through the compaction process and provide a casting to the required shape and dimensions. Furthermore the metal when cast into the casting mold, may contain casting defects, such as air bubbles, weakening the cast structure. If such defects cannot be rectified the cast is usually scrapped and the casting process repeated. It is desirable, therefore, to produce a casting model which can be used to produce several molds, if necessary. This is particularly advantageous when the desired shape is relatively large and has a relatively complex profile, such as, for example, automotive or `white goods` parts and body panels. If the casting model is not re-usable, the whole process must be repeated.
To provide a casting model with the required physical characteristics, the master model, fabricated and shaped using wood and plastics resin is usually used as the casting model. As such, the master model can be provided with the desired physical characteristics, but, as stated above, the wood and plastics resin materials are relatively difficult to work and relatively expensive to produce. Furthermore, in the automotive industry, design changes are frequently required for body and trim parts in view of the complexity of part shape and the interaction with adjacent parts. Such design changes in the final shape can be relatively slow to implement when the wood and plastics resin master model is used as the casting model.
It has also been considered to make a casting mold by shaping a body of low density plastics from ("STYROFOAM" trademark) to form a casting model, and forming a mold of molding sand against the shaped surface of the foam model. However, as the plastics foam used is a relatively open cellular structure, the molding sand sets into the surface cavities in the foam material, binding the casting model quite firmly to the sand mold. The low density foam casting model must therefore be left in place and allowed to burn out during the casting of the hot metal. This has a severe disadvantage in that toxic fumes, including cyanide gas, are produced as a result of the burn out step, which is not desirable from the view point of environmental protection. Furthermore, because the foam material is burnt out of the sand mold as the hot metal is poured in, contraflow of the molten metal and burn off products can cause impurities or voids in the cast metal. If the flow of the hot metal into the sand mold is too slow the metal can form localised relatively cool areas of skin and, therefore, not all of the foam material of the casting model may be burnt off, leaving impurities in the casting. If the flow of hot metal is relatively fast, as would be required to reliably ensure that all of the foam material is burnt off for large casting models, waves can be produced in the flowing material, causing it to flow over as it sets, leading to the formation of voids in the cast material. In practice, the formation of these impurities or voids is a limiting factor in the size of low density foam casting model which can be used. Also, as will be appreciated, the moldings and when compacted about the foam casting model, will flow into and fill the cellular cavities of the foam material which extend to the surface of the casting model. The surface of the sand mold at the sand/foam interface is therefore of relatively coarse texture and this surface is exposed when the foam of the casting model is burnt off. In view of the above, castings produced by the use of such low density foam casting models have a relatively poor surface finish which can require substantial localised `dressing` and general surface machining before being suitable for use in product manufacture. Additionally, if the actual casting is found to include impurities, air bubbles, or similar defects, causing weak points in the cast structure, the casting process has to be repeated. However, as the casting model has been destroyed by the actual casting process, a further casting model must be produced, with the attendant delays and additional costs to the overall process.
A further major disadvantage is that the low density foam model tends to deform when the molding sand is rammed against it.
The degree of deformation is non-uniform and can be relatively difficult to predict. If the shape is for use in manufacturing a high precision mold, additional tolerances must be allowed for in the cast shape, with the surplus material, which typically can be as much as 20 mm in thickness, being removed by a suitable machining technique. For `male` shapes the deformation of the casting model can be critical as there may be insufficient material in the actual cast shaper, i.e. the casting produced may be undersized, necessitating a repeat of the entire casting process, including the reproduction of the foam casting model.
The relative ease with which low density plastics foam can be deformed means that the material is unsuitable for use when the sand mold is relatively large and is therefore produced by high density ramming techniques in order to provide sufficient inherent strength to support the cast material.
In practice, therefore, low density foam casting models are limited to the production of relatively small items, typically having dimensions not exceeding 300 mm, such as automobile engine water pump housings.
In order to improve the deformation resistance of low density foam casting models it has also been considered to provide the foam material with a surface coating of relatively rigid material which may be applied, for example, by spraying. However, the coating does not provide sufficient rigidity for larger foam casting models as the foam material tends to collapse beneath the casting. Reinforcing structures, made of wood for example, may be used for incorporation into the foam material to minimise deformation during the compaction process. However, the composite wood/foam casting model is relatively difficult to produce and does not provide uniform rigidity.
Hence, such low density plastics foams are considered unsuitable for use in casting processes using relatively heavy metal alloys, such as zinc alloy, which, for large castings, would require inherently strong sand molds to support the cast material both during actual casting and during cooling down of the cast material. In particular, for zinc alloy the cooling down is usually performed under controlled conditions and over a relatively large period of time to ensure a high quality casting, so inherent strength in the sand mold is of paramount importance.
Additionally, as the low density foam is a relatively soft cellular material, it is difficult to machine with reasonable accuracy and surface finish by conventional rotary cutter machining techniques, for example by NC milling. The foam material tends to distort in front of the machine cutter, causing the material to tear away instead of cutting away cleanly, leaving an irregular finished surface with surface cavities. These difficulties in machining low density foam, coupled with the unpredictable compaction during the subsequent casting mold production process, means that NC data representative of the article to be manufactured would not be used in the production of a low density foam casting material. Such low density models are usually made by cutting and shaping by hand, using templates to achieve the final shape. However, as will be appreciated, the accuracy achievable is severely restricted. The above factors can add significantly to the overall cost of the casting process. Furthermore, because the NC data is not used in the production of the low density foam casting model, the data cannot be verified until a subsequent and more critical stage of the overall production process, for example, the machining of expensive graphite electrodes for use in electrical discharge machining (EDM) of the cast material.